The present invention relates to the art of induction heating and, more particularly, a method and apparatus for hardening axially spaced cams on a camshaft of the type used in internal combustion engines.
The invention is particularly applicable for inductively heating the axially spaced cams on an automobile engine camshaft formed from forged steel and it will be described with particular reference thereto; however, the invention has much broader applications and may be used for inductively heating the axially spaced cams on a variety of camshafts formed from various types of ferrous material.
Within the last few years, there has been a substantial demand for highly efficient, high performance internal combustion engines to be used in vehicles wherein the engine must have a reduced, combined rotational friction and be capable of operating at relatively high rotational velocities. Further, these engines must be reduced in weight and relatively simplified to thereby decrease the overall weight-to-horsepower ratio of the engine, so that fuel economy can be optimized. All of these commercial factors, together with foreign competitive situations, has resulted in a substantial amount of effort devoted to designing and manufacturing each component in the engine. One of the more critical components in an internal combustion engine is the rotating camshaft, which shaft has a plurality of axially spaced cams, each with a lobe that actuates a valve during operation of the engine. To decrease friction and increase rotational speed, it has been decided by many engine manufacturers to employ a roller follower adapted to contact the outer eliptical or eccentric surface of each cam to transmit the cam action to the valve itself. These roller followers exert a substantial force against the cam surface as the camshaft is rotated. In addition, due to the speed of operation, the cam surfaces must be very accurately controlled in dimensional characteristics to provide the efficient operation of the valve during engine operation. To accommodate the wear and provide dimensional accuracy, it has, in the past, been somewhat common practice to manufacture the camshaft from cast iron. The cams of the cast iron camshaft were then machined and hardened by first inductively heating the cam surface by surrounding inductor and then quench hardening the heated surface. To increase production, which is always an essential element of this type of equipment, devices such as those shown in U.S. Pat. Nos. 3,944,446 Bober and 3,784,780, Laughlin were developed by assignee of this application. Also, carbonized hardening processes were employed. Further, induction heating of all cams at the same time with plunge quenching was used for the purpose of heating the various camshafts. In these instances, the camshaft was rotated to provide uniformity. In addition, relatively low power densities were employed, below about 10 KW/in.sup.2, so that the hardened depth of the various cams was somewhat deep. All these processes required subsequent grinding and stoning for the purposes of generating the final cam surfaces, which must be done accurately. In other words, the hardened surfaces had to have a sufficient depth to facilitate actual generation of the cam surfaces subsequently by grinding and/or stoning. These post hardening processes necessitated relatively deep hardness and resulted in tools which needed dressing or changing after short periods of operation. All of these problems drastically increase the cost of the camshaft and often resulted in a cast iron hardened surface which was not sufficiently rigid to support high speed roller operations. This general situation required a re-thinking of camshaft technology for internal combustion engines of the type used in modern motor vehicles.
One of the solutions to the problems was to produce a forged steel camshaft blank which could be machined, inductively heated on the bearing surfaces of the cams, and then quench hardened, following by at least a stoning operation. The theory was, among other things, that the steel camshaft would have sufficient surface stability after hardening to operate in modern day motor vehicles. In addition, the steel camshafts could apparently be somewhat reduced in weight. All of these attributes contributed to the selection of this type of camshaft for use in the more recently manufactured engines for motor vehicles, at least for the United States market. Many people used the prior induction heating devices for the purpose of hardening the cams and also the fuel pump eccentric and distributor gear on the camshafts. Generally, the bearing surfaces concentric with the rotational axis of the camshaft could be left fairly soft since they presented a substantial area for support by axially spaced bearing blocks. Assignee of applicants, through applicants, embarked upon a revolutionary new approach for the purposes of hardening the surface of cams axially spaced along the camshaft, which process would overcome all of the disadvantages resulting from the mere attempt to use prior induction heating technology for these revolutionary new and technically complicated hardening problems. Applications of known technology did not produce camshafts at a production rate or quality meeting the present day demand. The present invention utilizes an approach described generally in the SAE technical paper series, No. 860231, entitled "Post Grind Hardening, an Alternative Method of Manufacturing a Steel Roller Camshaft".